In the drilling and completion of an oil or gas well, a cement composition is introduced to the well bore for cementing pipe string or casing in place. In this process, known as “primary cementing”, the cement composition is pumped into the annular space between the walls of the well bore and the casing. The cement composition sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath, which isolates the well bore from subterranean zones.
Changes in pressure or temperature in the well bore over the life of the well can produce stress on the cement composition, as can activities undertaken in the well bore, such as pressure testing, well completion operations, hydraulic fracturing, and hydrocarbon production. When these imposed stresses exceed the limiting strength of the cement, the cement sheath will fail and no longer provide zonal isolation. Compromised zonal isolation is highly undesirable, and necessitates remedial operations to be undertaken.
Due to its low tensile strength, neat cement is an undesirable material for use where there is a chance of expansive or tensile stresses developing in the well bore. Generally, a cement composition that has properties of elasticity and ductility, while retaining sufficient compressive strength and maintaining low permeability, is desirable.
Conventional cement is energy intensive. It is estimated that the cement industry produces approximately 5% of worldwide greenhouse gas (GHG) emissions. Total CO2 emissions from cement production range from 0.84 to 1.15 kg/kg clinker depending on production process and choice of raw materials. Given the limitations involved in reducing CO2 emissions from alternative raw materials and fuels, or by improving kiln efficiency, probably the most effective means of achieving significant reduction lies in the replacement of Portland cement clinker by other suitable materials. Supplementary cementitious materials such as fly ash, pozzolans, silica fume and metakaolin are commonly being used. However, these supplements have disadvantages. For example, the production of silica fume is also energy intensive and therefore it is an expensive product. Silica fume is also difficult and hazardous to handle.
Pumice cement was used by the ancient Greek and Roman and structures such as the Pantheon and the Coliseum were built with pumice cement. As pumice is inert and brittle, it can be used as an abrasive material for various purposes, including hand soaps, grill cleaners and skin removal products. Due to the cellular structure of pumice, which allows porosity but not permeability, it can be used as an absorbent (e.g., as a soil substitute and pesticide carrier). Pumice can also be ground into a powder to be used as filler in paints and asphalt mixes. Pumice has thermal insulation properties and compressive strength properties, which are useful in construction. Although pumice has been used in the concrete industry as a natural pozzolan, there is no report of its use in oil and gas industry for cementing oil and gas wells, as a result of the different properties desired for the materials used in the two industries.
There is a need in the art for a pumice-containing cement composition which is useful for cementing oil and gas wells.